/Engine/Plugins/Media/PixelStreaming/Source/PixelStreamingInput/Public/IPixelStreamingInputHandler.h
Register a custom function to execute when command JSON is received over the data channel: "{ type: "Command", YourCommand: YourCommandValue }". Note: You can also override the default Pixel Streaming command handlers by setting handlers with the same name as those already used, e.g. "Stat.FPS".
void SetCommandHandler ( const [FString](API\Runtime\Core\Containers\FString) & CommandName, const [TFunction](API\Runtime\Core\GenericPlatform\TFunction)< void([FString](API\Runtime\Core\Containers\FString), [FString](API\Runtime\Core\Containers\FString))> & Handler )